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A thick Quaternary alluvial section fills a sedimentary basin beneath the Santa Clara Valley, California, located within the San Andreas Fault system at the south end of San Francisco Bay. This section consists of an upper sequence about 1,000 feet thick containing eight sedimentary cycles and a lower finegrained unit as thick as several hundred feet. Together these constitute the Quaternary Santa Clara Basin. The section overlies an irregular unconformity with more than 1,200 feet of relief cut into the underlying bedrock. This stratigraphy is determined through study of new wells and seismic reflection profiles, together with a sample of the many thousands of water wells in the valley. It represents a major change and improvement in understanding of the basin, particularly with regard to the upper cyclic sequence, which forms a large groundwater system that is an important resource in the San Francisco Bay region. Each of the eight sedimentary cycles consists of a coarse-grained bottom interval overlain by a fine-grained top, with the coarse bottom forming a permeable sheet that is more or less continuous around the basin and the fine top forming a similarly extensive, relatively impermeable confining layer. This stratigraphic organization contrasts with most previous views, which have considered the coarse sediment in the basin to occur as scattered, discrete lenses and (or) sinuous channel sands, all embedded in a predominantly fine-grained section. Temperature logs in several wells demonstrate that the fine cycle tops do limit vertical movement of groundwater, although this may not be the case where those tops are thin to perhaps locally absent around parts of the basin margin. Age control has been obtained from previous work, in which the sedimentary cycles were correlated with the marine oxygen isotope record and the ages of two deeper Quaternary unconformities were estimated, and from detailed paleomagnetic study of cores from the new wells by E.A. Mankinen. Despite careful search of the cores, very few fossils were found, and none that are helpful in subdividing the section. No tephra (volcanic ash) was recovered, and the few carbon samples found and dated radiometrically are limited to the upper 120 feet of the section. The upper cyclic section ranges in age from 0 to somewhat older than 718 thousand years (ka), and the lower fine-grained section lies between unconformities with estimated ages of 950 and 1500 ka. Reflections in the seismic profiles indicate that layering in the basin is subparallel to the ground surface, and this fact, together with the continuous stratigraphic detail provided by geophysical logs of the new wells, allows the confident interwell correlation required to delineate the sedimentary cycles. The sequence of layers within any one cycle tends to persist laterally between the wells in the dataset, which are spaced 1 to 3 km apart, with most changes occurring gradually. The eight cycles, in contrast, tend to differ from each other in the details of their internal organization. Maps and cross sections show the elevations of cycle boundaries and the underlying bedrock surface, the varying thicknesses of the cycles and of their fine tops and coarse bottoms, and the aggregate thickness of coarse layers in those bottom intervals. Coarse sediment is more abundant toward some parts of the basin margin and in the southern part of the basin. Cycle boundary surfaces are relatively smooth, and their shapes are consistent with having been intercycle topographic surfaces. The underlying bedrock surface has a relief of more than 1,200 feet and deepens toward the center of the basin and the west edge of the fault-bounded Evergreen Basin, which is concealed beneath the east side of the Quaternary basin. The absence of consistent abrupt changes in thicknesses or boundary elevations across the basin or in cross section indicates that the interior of the basin is largely unfaulted, with the Silver Creek strand of the San Andreas system at the west edge of the Evergreen Basin being the sole exception. The east and west margins of the Santa Clara Basin, in contrast, are marked by reverse and thrust fault systems. Introduction Study of eight recently drilled deep wells in the alluvial fill of the Santa Clara Valley of northern California and associated data has led to discovery of an unanticipated regularity in the upper 1,000 feet of section. That regularity defines eight separate 2 Physical Subdivision and Description of the Water-Bearing Sediments of the Santa Clara Valley, California sedimentary cycles, which together constitute a large groundwater aquifer system that is a major source of water in the San Francisco Bay region. The primary goal of this report is to describe the sedimentary cycles and the sediment that forms them, together with the underlying bedrock unconformity and intervening earlier Quaternary section. Detailed hydrologic interpretation is not addressed, and the principal geologic description and interpretation are left for separate presentation. The study greatly improves understanding of the stratigraphy and structure of this alluvial basin and thus contributes to a better understanding of its aquifer system, earthquake response, and geologic history. The Santa Clara Valley1 is located within the California Coast Ranges between the San Andreas and Hayward Faults at the southern end of San Francisco Bay (fig. 1). The valley is floored by Quaternary deposits that overlie a large, subsiding Quaternary alluvial basin—the Santa Clara Basin (fig. 2). Many thousands of water wells have been drilled in the valley, and driller’s logs of those wells have been the principal source of information about the basin sediments. Additional information has now been obtained about the basin from eight new deep wells and other geophysically logged wells, as well as three shallow seismic reflection profiles, which together yield new insights about the basin and provide a guide to interpreting the preexisting wells. The material reported here is part of a large body of new work in the Santa Clara Valley that was stimulated jointly by the interest and support of the Santa Clara Valley Water District and projects funded by the National Geologic Mapping, Earthquake Hazards, and Water Resources Programs of the U.S. Geological Survey (USGS). The drilling, logging, and selective coring of the new wells in 2000–2003 were funded largely by the water district, and collection 1 Santa Clara Valley here refers to the lowland extending from Coyote Hills southeast to Coyote narrows (southeast of Oak hill) (see fig. 2). Caveras Fult an Adreas Fult Hayw rd Fult

Physical subdivision and description of the water-bearing sediments of the Santa Clara Valley, California